Metallurgical process of bending steel to desired curvature or straightness while avoiding losses in strength

ABSTRACT

A metallurgical process for bending steel bars or rods to the desired finished curvature or straightness without the normally expected losses in strength properties by bending the steel to straighten, etc., while at a temperature within the range of 300*-900* F. and preferably 400*-700* F. Steels which respond to such treatment are of the type which precipitation harden in response to such bending.

United States Patent [191 1 I $72,687 Gottschlich MIR larch 13, 1973 1 METALLURGICAL PROCESS OF FOREIGN PATENTS OR APPLICATIONS BENDING STEEL TO DESIRED CURVATURE OR STRAIGHTNESS WHILE AVOIDING LOSSES IN STRENGTH l,569,999 6/1969 France OTHER PUBLICATIONS Warm Working of Steel," Gokyu, et al., Translation Inventor; Rudolf Gottschlichr Griffith, Ind. of the Japan Institute of Metal, 1968, Vol. 9, Supple- [73] Assignee: La Salle SteelCompany, Hammond, 9 177481 1nd. I Primary Examiner-Lowell A. Larson [22] led: 1969 AttarneyMcDougall, Hersh, Scott & Ladd [211 App]. No.: 863,490

[57] ABSTRACT [52] U.S. Cl. ..72/364, 72/128, 148/12 A metallurgical process for bending steel bars or rods [51] Int. Cl. ..B21d 3/00 to the desired finished curvature or straightness Field of Search without the normally expected losses in strength pro- 148/12 perties by bending the steel to straighten, etc., while at a temperature within the range of 300-900 F. and [56] References Cited preferably 400-700 F. Steels which respond to such UNITED STATES PATENTS treatment are of the type which precipitation harden in response to such bending. 2,880,855 4/1959 Nachtman ..72/364 3,573,999 4/1971 Isao Gokyu ..l48/l2 6 Claims, 2 Drawing Figures I l 156-- 76725116 I I 'T S l l I 140" l i//"" I 1 T l-\ Yi ld {Q l I i 1 l 12 I30-- l l m 4 i i k l l l k I l i I 120 l I l\ l Q i O 11 l l Q \I u if 1 81.11 /04! $2eel I Drawn 0377 fmzsh Szze ii/ 5 l l I I l l t l l i y l i l AD RT 200 300 400 500 600 700 800 900 STRAIGHTEN/A/G TEMP.

METALLURGICAL PROCESS OF BENDING STEEL TO DESIRED CURVATURE OR STRAIGHTNESS WHILE AVOIDING LOSSES IN STRENGTH This invention relates to the production of finished steel bars and rods and more particularly to the metallurgical process of bending the steel bars or rods to effect the desired curvature or straightness while avoiding the normally expected deteriorations in strength properties.

Steel bars and rods which have been processed for reduction in cross-sectional area by drawing, extrusion, rolling, forging, shaving or machining and the like rarely meet the requirements of commercial straightness. The processed bars and rods are usually advanced through a rotary straightener as a final finishing step to eliminate the undesirable curvature of an as processed, such as an undrawn, bar or rod and to produce a finished product within commercial tolerances.

To achieve the desired straightness, the as processed bar or rod must be plastically deformed by bending as it passes between the straightening rolls. In so doing, the residual stress pattern of the steel bar or rod is altered. This modifies the strength and ductility properties of the as processed" steel, generally depressing the yield and tensile strength values with but minor changes in elongation and reduction of area. These changes are difficult to control and, as a result, finished steel bars and rods usually have a wide range of variation in tensile properties.

An object of this invention is to provide a metallurgical process wherein steel bars or rods are subjected to bending to the desired curvature or straightness without the normally expected losses in strength properties. An important concept of the invention resides in the avoidance of losses in strength properties of the steel as a result of such straightening operations.

It has been found, in accordance with the practice of this invention, that the losses in strength properties in steel bars and rods can be avoided when the steel bars or rods are subjected to bending operations to straighten or to a desired curvature while the steel is at a temperature within the range of 300900 F. and preferably at a temperature within the range of 400-700 F.

As used herein, the term straightening" is meant not only to include the conventional straightening operations by processing the steel bars or rods between bending rolls to achieve the straightness desired in the bar or rod but also to include such other metallurgical means for bending the steel to achieve a desired curvature of a finished product. Such processing of steel bars and rods to finished shape generally involves the bending of the steel to beyond its elastic limit to produce the desired degree of straightness or curvature. Such straightening operations can be achieved in a number of ways, such as by processing the steel bars or rods as by means of straightening rolls, punches, anvil, arbors, or by using bending dies.

The described phenomenon of retention of strength properties, instead of decrease in strength properties, during straightening, is not experienced with all steels. It is experienced only with steels that strain or work harden and which precipitation harden when worked in the described manner, such as by the precipitation of carbon and nitrogen atoms along substructural defects, referred to generally as dynamic strain aging. Such characteristics are present in low and medium carbon steels and particularly non-austenitic steels having a pearlitic structure in a matrix of either free ferrite or bainite. Steels having little or no free ferrite in their structure appear to be incapable of retention of strength in response to straightening at elevated temperature within the range described. 0n the other hand, non-austenitic steels containing substantial amounts of free ferrite are capable of retaining their strength properties while at a temperature within the range of 300900 F. and preferably 400-700 F.

The steel bar or rod should be heated to the desired temperature for straightening immediately prior to processing with the straightening rolls, punch, anvil, bending dies or the like. This can be accomplished by advancing the steel through an induction heater aligned with the straightening machine immediately in advance of the feed opening; it can be accomplished by immersing the steel bar or rod into a molten salt or metal bath, such as a molten lead bath, immediately prior to feeding the steel to the straightening machine; it can be provided with steels which have previously been processed at elevated temperature higher than the desired straightening temperature, by controlling the cooling down of the steel to the desired temperature level for immediate processing through the straightening machine. Heating of the steel to the desired temperature can be achieved in a number of ways, but it is desirable to avoid prolonged heating or slow heating which might otherwise interfere with the surface characteristics of the steel or materially modify the physical and mechanical properties thereof.

After the warm straightening, the finished steel can be allowed to cool down normally or it can be cooled down more rapidly by a liquid or air quench.

FIGS. 1 and 2 are'curves illustrating the improved yield and tensile properties achieved by straightening within the temperature range of the invention.

Having described the basic concepts of the invention, illustration will now be made by way of the following examples, which are given by way of illustration, but not by way of limitation:

EXAMPLE 1 Steel Composition (1041 steel): 0.39 percent carbon 1.50 percent manganese 0.013 percent phosphorus 0.025 percent sulphur 0.280 percent silicon 0. 10 percent chromium 0.l0 percent nickel 0.03 percent molybdenum 0.l0 percent copper 0.0 l 0 percent nitrogen balance iron EXAMPLE 2 Steel Composition (lOl 1 steel): 0.08 percent carbon 0.75 percent manganese 0.12 percent phosphorus 0.27 percent sulphur 0.01 percent silicon 0.015 percent nitrogen balance iron EXAM PLE 3 Steel Composition (1052 steel): 0.48 percent carbon 1.50 percent manganese 0.03 percent phosphorus 0.27 percent sulphur 0.30 percent silicon 0.005 percent nitrogen balance iron EXAMPLE 4 Steel Composition 101 8 steel):

0.17 percent carbon 0.75 percent manganese 0.03 percent phosphorus 0.04 percent sulphur 0.08 percent silicon 0.005 percent nitrogen balance iron As a starting material, use is made of the steel of Example in the form of drawn rods which were drawn down to a diameter of 0.377 inch at a temperature of 700 F. The coil of drawn steel was subdivided into a number of strands. Each strand was passed through a conventional five-roll straightening mill at a different temperature ranging from room temperature to a maximum of 900 F. The strands processed at a temperature in excess of 300 F. were heated in a molten lead bath to the desired temperature and then immediately fed to the straightener to avoid heat losses. After cooling to room temperature, a number of specimens (four) were cut from spaced portions along the length of each of the strands and machined for testing in tension.

The results secured, as an average of each of the four samples, are illustrated in FIG. 1, which relates yield or tensile stress to straightening temperature.

It will be seen that increase in the straightening temperature for the steel results in rapid increase in yield and tensile strength of the straightened material within the temperature range of 400-600 F. by comparison with straightening at room temperature. While the strength properties tend to fall off-at straightening temperatures beyond 600 F., the values secured are still higher than those of the as drawn rod or the as drawn rod subjected to cold straightening at room temperature or at temperatures below 300 F.

Similar results were secured with medium carbon resulphurized steels as represented by ll44 steel and any of the other steels of Examples 2-4.

The as drawn" rod was subdivided into strands and each strand was subjected to straightening in a roll straightener in the manner described in Example l, at different temperatures ranging from room temperature to 600 F. and then allowed to cool down normally in air. Specimens were taken and tested as in Example 1 with the results illustrated in FIG. 2 attached hereto and made a part hereof.

As with the medium carbon, high manganese steel, the medium carbon re-sulphurized steel gave increased tensile and yield strengths in response to straightening at temperatures within the range of 400-600 F. The increase in tensile and yield strength was achieved with steels of the type described without noticeable loss in elongation or reduction in area. I

It Will be apparent from the foregoing that not only 15 the steel straightened by bending without losses in strength properties but, in many instances, increase in strength properties is unexpectedly experienced in response to such warm straightening of the steel in accordance with the practice ofthis invention.

it will be understood that changes may be made in the details of composition and operation without departing from the spirit of the invention, especially as defined in the following claims.

lclaim:

1. in the method of straightening steels which precipitation harden and without loss in strength properties, the steps of bending a steel bar or rod beyond its elastic limit to straighten or to the desired curvature without noticeable change in dimension while the steel is at a temperature within the range of 300-900 F.

2. The method as claimed in claim 1 in which the steel is a pearlitic steel having a matrix of free ferrite or bainite.

3. The method as claimed in claim 1 in which the steel is in the form of bars, rods or strip.

4. The method as claimed in claim 1 in which the steel is bent to straighten while at a temperature within the range of 400-700 F.

5. The method as claimed in claim 1 in which the steel is straightened by advancing the steel between straightening rolls.

6. The method as claimed in claim 1 in which the steel is an as processed steel.

* i I? t 

1. In the method of straightening steels which precipitation harden and without loss in strength properties, the steps of bending a steel bar or rod beyond its elastic limit to straighten or to the desired curvature without noticeable change in dimension while the steel is at a temperature within the range of 300*-900* F.
 2. The method as claimed in claim 1 in which the steel is a pearlitic steel having a matrix of free ferrite or bainite.
 3. The method as claimed in claim 1 in which the steel is in the form of bars, rods or strip.
 4. The method as claimed in claim 1 in which the steel is bent to straighten while at a temperature within the range of 400*-700* F.
 5. The method as claimed in claim 1 in which the steel is straightened by advancing the steel between straightening rolls. 